update Dockerfile and add note in README.md

Dockerfile

@@ -1,32 +1,22 @@
-FROM alpine
+FROM alpine:3.16
 
 MAINTAINER Niklas Elsbrock
 
-# add apk edge repositories (including `testing` for py3-flask-sqlalchemy)
-# and update package index
-RUN echo -e "https://dl-cdn.alpinelinux.org/alpine/edge/main/\n"\
-"https://dl-cdn.alpinelinux.org/alpine/edge/community/\n"\
-"https://dl-cdn.alpinelinux.org/alpine/edge/testing/"\
-	> /etc/apk/repositories \
-	&& apk --no-cache update
+RUN echo "https://dl-cdn.alpinelinux.org/alpine/edge/testing/" >> /etc/apk/repositories \
+    && apk add --no-cache \
+        python3 \
+        py3-flask \
+        py3-flask-sqlalchemy \
+        py3-click \
+        py3-yaml \
+        py3-bcrypt \
+        py3-openpyxl \
+    && mkdir -p /srv/nateman/app
 
-# install nateman
-RUN apk --no-cache add \
-	python3 \
-	py3-flask \
-	py3-flask-sqlalchemy \
-	py3-click \
-	py3-yaml \
-	py3-bcrypt \
-	py3-openpyxl \
-	&& mkdir -p /srv/nateman/nateman
-
-COPY nateman /srv/nateman/nateman
+COPY nateman /srv/nateman/app
 WORKDIR /srv/nateman
 
-# expose webserver port
 EXPOSE 8080
 
-# start command
-CMD ["env","FLASK_APP=nateman","FLASK_ENV=development",\
-	"flask","run","--host=0.0.0.0","--port=8080"]
+ENV FLASK_ENV=development
+CMD ["flask","run","--host=0.0.0.0","--port=8080"]